The present invention relates to a lubricating device for telescopic shaft with axial conduits formed in its relatively sliding tubular parts. The telescopic shaft is, in particular, a cardanic shaft for transmitting power from a force outlet of an agricultural tractor to an operating machine or driven tool.
In general, the cardanic shaft comprises an intermediate shaft made of two tubular parts bearing, at the respective free extremities, a fork for cardanic joint or the like. The tubular parts of the intermediate shaft, an outer one and an inner one, have conformal profiles in order to be able to slide relatively one inside the other, but are prevented from rotating relative to one another. The cardanic shaft can adapt itself to the mutual displacements of the tractor and of the operating machine or driven tool thanks to the mutual displacements of the tubular parts which allow it to be lengthened and shortened.
This lengthening and shortening adaptation takes place under a generally high transmission torque and hence generates friction on the contact surfaces of the tubular parts of intermediate shaft. The consequences thereof are heating and wear of the aforesaid parts, with obvious consequences of a reduced working life due to defects and breakages.
Lubricating devices mounted on a cardanic shaft have already been realized, with the aim of avoiding the assembling and re-assembling operations which would be necessary for the periodic lubrication of the inner surface of the outer tubular part or of the outer surface of the inner tubular part.
Among others, Italian patent No. 1,286,359 by the same applicant discloses a lubricating device for telescopic shaft, comprising a ball greaser mounted in the fork of the inner tubular part of the telescopic shaft and connected, internally thereto, with a small connecting tube to an element for the storage and distribution of the grease, positioned inside the inner tubular part and having an inner cavity communicating, through its own openings and corresponding through holes obtained in the inner tubular part, with the inner surface of the outer tubular part of the intermediate shaft.
Thus, such a device is able from the exterior to let the grease arrive between the inner and outer tubular portions of a telescopic tube without disassembling any component of the transmission. However, since the play or inter-space between the two tubular parts of the telescopic shaft is relatively small, generally ranging between 0.5 and 0.9 mm, the grease, due to its viscosity, is not able to flow axially in that inter-space, except for a few centimeters. This is due to the fact that resistance to sliding increases extremely rapidly, so that the operator with a manual injector is no longer able to pump grease into the interior. Thus, the quantity of grease that enters the inter-space between the two tubular parts of the telescopic shaft is so small as to cover a very reduced surface area relative to the area of actual sliding, with the consequence of an insufficient lubrication.
On the other hand, if a motorized lubricating apparatus were to be used, whence grease exits at a sufficiently high pressure for an effective diffusion in that inter-space, such pressure could jeopardize the soundness of the couplings in the connecting tube, which is usually made of plastic material.
Moreover, even if the connecting pipe were made for high pressures, motorized lubricating apparatuses may periodically not be available in place.